Pwned uses the Have I Been Pwned? database created by security researcher Troy Hunt. When sites are hacked, any public information about the accounts and passwords included in that breach are added to this database. It can include user names, email addresses, passwords, and more.
You can use Pwned to check if your account or passwords have been a part of a data breach. If so, change your passwords for the relevant accounts and do not reuse that password.